What is PAR? Is this the same as Ambient Light?

FAQ - Li-Cor Ambient Light

1. Q: What is PAR? Is this the same as Ambient Light?A: PAR stands for Photosynthetically Active Radiation. PAR is a specific type of Ambient Light. Hydrolab's Ambient Light sensor is also a PAR sensor.
   
   
2. Why would I want to measure PAR?
3. Under what circumstances would I use the Flat-PAR sensor instead of the Spherical-PAR sensor?
4. How is the PAR sensor calibrated?
5. Under what circumstances would I use the Dual-PAR sensor instead of the Single-PAR sensor?